Influence of stress simulation parameters on the fracture strength of all-ceramic fixed-partial dentures
Introduction
In vitro studies help develop dental materials, but only the experience of the dental scientist allows evaluation of the results to estimate the principal applicability of the material under clinical conditions. In fact, only a clinical trial implies a final warranty of a new material. Nevertheless, in vitro tests, which simulate the clinical conditions allow a first efficient estimation of the material in the shape of a dental restoration. Indeed it is necessary to correlate the in-vitro testing with the clinical experience-but well performed clinical trials are rare.
Various simulation devices and their modifications were introduced, e.g. by DeLong and Douglas [1], [2], Krejci et al. [3], [4] and others [5], [6], [7], [8]. Partly these machines are commercially (EGO, G; EnduraTEC, USA; Willytech, G; SDE, USA) available. The devices try to approach the clinical situation by simulating oral parameters such as chewing force (amount, frequency), thermal loading, moisture, lateral jaw motion, human abutments, periodontium or antagonistic denture. Beyond this, some devices vary the shape of the applied force (sinusoidal, rectangular) or simulate an increasing force (staircase) for Weibull statistics. Special devices were constructed to simulate wear on specimens (Three-body wear ACTA [9], Pin-on block/disc, OHSU), and some ‘artificial mouths’ allow to imitate attrition and abrasion wear in combination with food bolus on dental restorations.
The mastication devices simulate mastication loading (ML), some devices in combination with simultaneous thermal cycling (TC). The force is applied with weight, compressed air or via electric motor and varies between 12 and 70 N. The thermal loading is generated with distilled water of one temperature (37 °C). When temperature changes were required, the testing chambers were floated with cold and hot water (e.g. 5 and 55 °C), or more progressive, the water is continuously circulated to guarantee constant temperatures. The generally short exposure to the moisture environment (less than 10 days) allows only a minor diffusion and therefore marginally contributes to the aging process.
Unfortunately, there are no generally accepted TCML parameters and standardization is out of reach. It is known, that variations of loading frequency [10], [11] or the type of abutment material [12], [13] contribute to the aging process, but it is unclear what consequences were to be expected using different adjusted simulation parameters. Beyond this, the question arises, whether different TCML devices show comparable aging results. The aim of this investigation was to compare the influence of different simulation parameters on the properties of dental restorations. All ceramic FPDs were investigated as an example of dental restorations for their high susceptibility by cracking, torque and bending [10], [14]. The FPDs were aged with different adjustments or TCML devices and their resistance to occlusal loading before and after aging was determined. The results were compared to available clinical data.
Section snippets
Material and methods
Artificial and human molar teeth were inserted in a polymethylmethacrylate resin with an approximal distance between the teeth of 10 mm, representing a molar gap. The preparation was a circular 1 mm deep chamfer finishing line. 14 groups of all-ceramic three-unit Empress 2 FPDs, each consisting of eight bridges, were fabricated according to the manufacturer's instructions. The inside of the FPD abutments were etched with hydrofluoric acid, covered with the silane-coupling agent (Monobond-S) and
Results
The significantly highest fracture resistance (1832+/−342 N) was found for the control group without aging. Only the results after mechanical loading with thermal cycling at a constant temperature (11) did not affect the fracture load. With increasing loading force from 50 N (1) to 150 N (2) the loading capacity of the FPDs was significantly reduced. A staircase load-increase to 150 N (7) showed no significant differences compared to a test with a constant load of 150 N (2). Whether applying the
Discussion
Variations of the loading parameters lead to different results of fracture force after artificial aging. The highest force reduction of about 70% was achieved using ‘standard’ settings with human or polymeric (liquid cristal polymer=LCP) abutment teeth. The application of the alloy abutments clearly lead to an overestimation of the ceramic FPDs. LCP ensures a dentin-like modulus of elasticity and allows for the preparation of identical abutment dimensions [13], but beyond this, evaluations of
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